c. Lowest level. The lowest level of light units may be installed at a higher elevation than normal on a structure if the surrounding terrain, trees, or adjacent building(s) would obscure the lights. In certain instances, as determined by the FAA, the lowest level of lights may be eliminated.
5.3 Control Device.
Red obstruction lights should be operated by an acceptable control device (e.g., photocell, timer, etc.) adjusted so the lights will be turned on when the northern sky illuminance reaching a vertical surface falls below a level of 60 foot-candles (645.83 lux) but before reaching a level of 35 foot-candles (376.73 lux). The sensing device should, if practical, face the northern sky in the Northern Hemisphere (see AC 150/5345-43, Specification for Obstruction Lighting Equipment).
The control device should turn the lights off when the northern sky illuminance rises to a level of not more than 60 foot-candles (645.83 lux). The lights may also remain on continuously.
5.4 Alternate Method of Displaying Obstruction Lights.
In certain cases, instead of installing lights on the obstruction, the FAA may recommend the placement of a light(s) on an adjacent pole of equal height.
5.5 Poles, Towers, and Similar Skeletal Structures.
The following standards apply to radio and television towers, supporting structures for overhead transmission lines, and similar structures.
1. Top-Mounted Obstruction Lights.
a. Structures 150 feet (45.72 m) AGL or less. Two or more steady-burning red (L- 810) lights should be installed in a manner to ensure an unobstructed view of one or more lights by a pilot.
b. Structures exceeding 150 feet (45.72 m) AGL. At least one red flashing (L- 864) light should be installed in a manner to ensure an unobstructed view of one or more lights by a pilot.
c. Appurtenances 40 feet (12.19 m) or less. If a rod, antenna, or other appurtenance 40 feet
(12.19 m) or less in height is incapable of supporting a red flashing light, then it may be placed at the base of the appurtenance. If the mounting location does not allow an unobstructed view of the light by a pilot approaching in any direction, then additional lights should be added.
d. Appurtenances exceeding 40 feet (12.19 m). If a rod, antenna, or other appurtenance exceeding 40 feet (12.19 m) in height is incapable of supporting a red flashing light, a
supporting mast with one or more lights should be installed adjacent to the appurtenance.
Adjacent installations should not exceed the appurtenance’s height and be within 40 feet
(12.19 m) of the tip to allow the pilot an unobstructed view of at least one light. If the rod, antenna, or other appurtenance is 7/8 inch wide or more, at least two lights must be installed on the supporting mast to provide the necessary unobstructed view.
2. Mounting Intermediate Levels. The number of light levels is determined by the height of the structure, including all appurtenances, as shown in , Figure A-6. The number of lights on each level is determined by the shape and height of the structure. These lights should be mounted to ensure an unobstructed view of at least one light by a pilot approaching in any direction.
a. Steady-burning lights (L-810).
i. Structures 150 feet (45.72 m) AGL or less. Two or more steady-burning lights should be installed diagonally or on diametrically opposite positions.
ii. Structures exceeding 150 feet (45.72 m) AGL. These structures do not require steadyburning lights.
b. Flashing lights (L-810 F). For structures exceeding 150 feet (45.72 m) but not more than 350 feet (106.68 m), two or more flashing lights should be mounted outside at diagonally opposite positions at intermediate levels. These lights should be configured to flash simultaneously with the L-864 flashing light on the top of the structure at a rate of 30 flashes per minute (fpm) (± 3 fpm).
c. Flashing lights (L-864).
i. Structures 350 feet (106.68 m) AGL or less. These structures do not require flashing (L-864) lights at intermediate levels.
ii. Structures exceeding 350 feet (106.68 m) AGL. At intermediate levels, two (L-864) lights should be mounted outside at diagonally opposite positions.
5.6 Chimneys, Flare Stacks, and Similar Solid Structures (except Hyperbolic Cooling Towers).
5.6.1 Number of Light Units.
The number of units recommended depends on the diameter of the structure at the top. The number of lights recommended below is the minimum (see Figure A- 10).
1. Structures 20 feet (6.10 m) or less in diameter. Three light units per level.
2. Structures exceeding 20 feet (6.10 m) but not more than 100 feet (31 m) in diameter. Four
light units per level.
5.6.2 Top-Mounted Obstruction Lights.
1. Structures 150 feet (45.72 m) AGL or less. L-810 lights should be installed horizontally at regular intervals at or near the top.
2. Structures exceeding 150 feet (45.72 m) AGL. At least three L-864 lights should be installed.
3. Chimneys, Cooling Towers, and Flare Stacks. Lights may be displayed as low as 20 feet
(6.10 m) below the top (, Figure A-7) to avoid the obscuring effect of deposits and heat generally emitted by this type of structure. It is important that these lights are readily accessible for cleaning and lamp replacement. It is understood that with flare stacks, as well as any other structures associated with the petrol-chemical industry, normal lighting requirements may not be necessary. This could be due to the location of the flare stack/structure within a large, well-lighted, petrol-chemical plant, or the fact that the flare, or working lights surrounding the flare stack/structure, is as conspicuous as obstruction lights.
5.6.3 Mounting Intermediate Levels.
The number of light levels is determined by the height of the structure including all appurtenances. Structures between 150 feet and 350 feet (45.72 m and 106.68 m) AGL should have a second level of steady-burning red light units installed approximately at the midpoint of the structure and in a vertical line with the top level of lights. Structures exceeding 350 feet
(106.68 m) AGL should have a second level of flashing light units. For cooling towers 600 feet
(182.88 m) AGL or less, intermediate light levels are not necessary.
1. Steady-burning (L-810) lights. The recommended number of light levels is shown in Figure A-6. At least three lights should be installed on each level.
2. Flashing (L-864) lights. The recommended number of light levels is shown in Figure A-6.
a. Structures 350 feet (106.68 m) AGL or less. These structures do not need intermediate levels of flashing lights.
b. Structures exceeding 350 feet (106.68 m) AGL. At least three flashing (L-864) lights should be installed on each level in a manner allowing an unobstructed view of at least one light.
5.7 Prominent Buildings, Bridges, and Similar Extensive Obstructions.
When objects within a group of obstructions are approximately the same overall height above the surface and are located a maximum of 150 feet (45.72 m) apart, the group of obstructions may be considered an extensive obstruction. Light units should be installed on the same horizontal plane at the highest portion, or edge, of the prominent obstructions. Light units should be placed to
ensure the light is visible to a pilot approaching from any direction. If the structure is a bridge (see Figure A-8) and is over navigable water, the sponsor must obtain prior approval of the lighting installation from the Commander of the District Office of the U.S. Coast Guard to avoid interference with marine navigation. Steady-burning lights should be displayed to indicate the extent of the obstruction as follows:
1. Structures 150 feet (45.72 m) or less in any horizontal direction. If the structure/bridge/extensive obstruction is 150 feet (45.72 m) or less horizontally, at least one steady-burning light (L-810) should be displayed on the highest point at each end of the obstruction’s major axis. If this is impractical because of the overall shape, display a double obstruction light in the center of the highest point.
2. Structures exceeding 150 feet (45.72 m) in at least one horizontal direction. If the structure/bridge/extensive obstruction exceeds 150 feet (45.72 m) horizontally, at least one steady-burning light should be displayed for each 150 feet (45.72 m), or fraction thereof, of the overall length of the major axis. At least one of these lights should be displayed on the highest point at each end of the obstruction. Additional lights should be displayed at approximately equal intervals, not to exceed 150 feet (45.72 m) on the highest points along the edge between the end lights. If an obstruction is located near a landing area and two or more edges are the same height, the edge nearest the landing area should be lighted.
3. Structures exceeding 150 feet (45.72 m) AGL. Steady-burning red obstruction lights should be installed on the highest point at each end. At intermediate levels, steady-burning red lights should be displayed for each 150 feet (45.72 m), or fraction thereof. The vertical position of these lights should be equidistant between the top lights and the ground level, as the shape and type of obstruction will permit. A steady-burning red light should be displayed at each outside corner on each level with the remaining lights evenly spaced between the corner lights.
4. Exceptions. Flashing red lights (L-864) may be used instead of steady-burning lights if early or special warning is necessary. These lights should be displayed on the highest points of an extensive obstruction at intervals not exceeding 3,000 feet (914.40 m). At least three lights should be displayed on one side of the extensive obstruction to indicate a line of lights.
5. Ice Shields. Where icing is likely to occur, metal grates or similar protective ice shields should be installed directly over each light unit to prevent falling ice or accumulation from damaging the light units. The light should be mounted in a manner to ensure an unobstructed view of at least one light by a pilot approaching from any direction.
5.8 Group of Obstructions.
With the exception of wind turbines, when individual structures within a group of obstructions differ in height and are spaced no more than 150 feet (45.72 m) apart, the prominent structures within the group should be lighted in accordance with the standards for individual obstructions
based on its corresponding height. Shorter structures within the group of obstructions do not need to be lighted. When structures are shorter than the prominent structure and are located on the outside of the group of obstructions, those structures should be lighted in accordance with the standards for individual obstructions based on its corresponding height. In addition to lighting the shorter structures on the outside of the group, at least one flashing light should be installed either at the top of the tallest, most prominent center structure, or on a dedicated tower that is located near the center of the group and is the same height as the most prominent structure. Light units should be placed on the structures to ensure that the lights are visible to a pilot approaching from any direction. If one or more of the structures within the group are a solid mass (nonskeletal), additional lighting may be necessary to make sure that the light is not being blocked by the more prominent structure(s). For the purpose of marking and lighting these structures, a group of obstructions is considered to be three or more structures.
CHAPTER 6. MEDIUM-INTENSITY FLASHING WHITE OBSTRUCTION LIGHT
SYSTEMS
6.1 Purpose.
Medium-intensity flashing white (L-865) obstruction lights may provide conspicuity both day and night. Recommendations on lighting structures can vary, depending on terrain features, weather patterns, geographic location, and number of structures.
Using a 24-hour, medium-intensity, flashing white light system in urban/populated areas is not normally recommended due to their tendency to blend with the background lighting in these areas at night. This makes it extremely difficult for some types of aviation operations, i.e., medical-evacuation (medevac) and police helicopters to see these structures. Using this type of system in urban and rural areas often results in complaints. In addition, this system is not recommended on structures within 3 NM of an airport.
6.2 Standards.
The medium-intensity flashing white light system is normally composed of flashing omnidirectional lights. Medium-intensity flashing white obstruction lights may be used during daytime and twilight with automatically selected, reduced intensity for nighttime operation.
When this system is used on structures 700 feet (213.36 m) AGL or less, other methods of marking and lighting the structure may be omitted. Aviation orange and white paint is always required for daytime marking on structures exceeding 700 feet (213.36 m) AGL. This system is not normally recommended on structures 200 feet (60.96 m) AGL or less. The number of light levels needed is shown in Figure A-9.
6.3 Control Device.
The light intensity is controlled by a device (photocell) that changes the light’s intensity when the ambient light changes. The illuminance sensing device should, if practical, face the northern sky in the Northern Hemisphere and the system should automatically change intensity steps when the illumine sense reaching a north-facing vertical surface is as follows:
1. Twilight-to-Night. This should not occur before the illumination drops below 5 foot-candles
(53.82 lux) but should occur before it drops below 2 foot-candles (21.53 lux).
2. Night-to-Day. The intensity changes listed in subparagraph 6.3 1 above should be reversed when changing from the night-to-day mode.
6.4 Radio and Television Towers and Similar Skeletal Structures.
6.4.1 Mounting Lights.
The number of levels recommended depends on the height of the structure, including antennas and similar appurtenances.
1. Top levels. One or more lights should be installed at the highest point to provide 360-degree coverage, ensuring an unobstructed view by a pilot approaching from any direction.
2. Appurtenances 40 feet (12.19 m) or less . If a rod, antenna, or other appurtenance 40 feet
(12.19 m) or less in height is incapable of supporting the medium -intensity flashing white light, then it may be placed at the base of the appurtenance. I f the mounting location does not allow an unobstructed view of the medium -intensity flashing white light by a pilot approaching from any direction, then additional lights should be added.
3. Appurtenances exceeding 40 feet (12.19 m). If a rod, antenna, or other appurtenance exceeds 40 feet (12.19 m) above the tip of the main structure, a medium -intensity flashing white light should be placed within 40 feet (12.19 m) from the top of the appurtenance. If the appurtenance (such as a whip antenna) is incapable of supporting the light, one or more lights should be mounted on a pole adjacent to the appurtenance. Adjacent installations should not exceed the height of the appurtenance and be within 40 feet (12.19 m) of the tip to allow the pilot an unobstructed view of at least one light. If the rod, antenna, or other appurtenance is 7/8 of an inch wide or more, at least two lights must be installed on the supporting mast to provide the necessary unobstructed view.
6.4.2 Mounting Intermediate Levels.
At intermediate levels, two or more lights (L-865) should be mounted outside at diagonally or diametrically opposite positions of intermediate levels. The lowest light level should not be less than 200 feet (60.96 m) AGL.
6.4.3 Lowest Levels.
The lowest level of light units may be installed at a higher elevation than normal on a structure if the surrounding terrain, trees, or adjacent building(s) would obscure the lights. In certain instances, as determined by the FAA, the lowest level of lights may be eliminated.
6.4.4 Structures 700 feet (213.36 m) AGL or less.
When medium-intensity flashing white lights are used during nighttime and twilight only, marking is required for daytime. When operated 24 hours a day, other methods of marking and lighting are not required.
6.4.5 Structures exceeding 700 feet (213.36 m) AGL.
Medium intensity lights should be used during nighttime, twilight, and may be used 24 hours a day. Additionally, marking is always required for daytime. The number of light levels needed is the same as high intensity lights as shown in Figures A-13 and A-14.
6.4.6 Ice Shields.
Where icing is likely to occur, metal grates or similar protective ice shields should be installed directly over each light unit to prevent falling ice or accumulation from damaging the light units.
The light should be mounted in a manner to ensure an unobstructed view of at least one light by a pilot approaching from any direction.
6.5 Chimneys, Flare Stacks, and Similar Solid Structures.
The recommended number of top-level light units depends on the diameter of the structure at the highest point of a structure. The lights should be installed at the highest point, however, top level chimney lights may be installed as low as 20 feet (6.10 m) below the top to minimize deposit build-up due to emissions (see Figure A-10). The number of lights per level below is the minimum recommended.
1. Structures 20 feet (6.10 m) or less in diameter. Three light units per level
2. Structures exceeding 20 feet (6.10 m) but not more than 100 feet (31 m) in diameter. Four light units per level.
6.6 Prominent Buildings and Similar Extensive Obstructions.
With the exception of wind turbines, when objects within a group of obstructions are approximately the same overall height above the surface and are located a maximum of 150 feet
(45.72 m) apart, the group of obstructions may be considered an extensive obstruction. Light units should be installed on the same horizontal plane at the highest portion, or edge, of the prominent obstructions. Light units should be placed to ensure that the light is visible to a pilot approaching from any direction. Lights should be displayed to indicate the extent of the obstruction as follows:
1. Structures 150 feet (45.72 m) or less in any horizontal direction. If the structure/extensive obstruction is 150 feet (45.72 m) or less horizontally, at least one light should be displayed on the highest point at each end of the obstruction’s major axis. If this is impractical because of the overall shape, display a double obstruction light in the center of the highest point.
2. Structures exceeding 150 feet (45.72 m) in at least one horizontal direction. If the structure/extensive obstruction exceeds 150 feet (45.72 m) horizontally, at least one light should be displayed for each 150 feet (45.72 m), or fraction thereof, of the overall length of the major axis. At least one of these lights should be displayed on the highest point at each end of the obstruction. Additional lights should be displayed at approximately equal intervals not to exceed 150 feet (45.72 m) on the highest points along the edge between the end lights. If an obstruction is located near a landing area and two or more edges are the same height, the edge nearest the landing area should be lighted.
3. Structures exceeding 150 feet (45.72 m) AGL. Lights should be installed on the highest point at each end. At intermediate levels, lights should be displayed for each 150 feet
(45.72 m), or fraction thereof. The vertical position of these lights should be equidistant between the top lights and the ground level as the shape and type of obstruction will permit. One such light should be displayed at each outside corner on each level with the remaining lights evenly spaced between the corner lights.
6.7 Group of Obstructions.
With the exception of wind turbines, when individual structures within a group of obstructions differ in height and are spaced no more than 150 feet (45.72 m) apart, the prominent structures within the group should be lighted in accordance with the standards for individual obstructions based on its corresponding height. Shorter structures within the group of obstructions do not need to be lighted. When structures are shorter than the prominent structure and located on the outside of the group of obstructions, those structures should be lighted in accordance with the standards for individual obstructions based on its corresponding height. In addition to lighting the shorter structures on the outside of the group, at least one flashing light should be installed either at the top of the tallest, most prominent center structure, or on a dedicated tower that is located near the center of the group and is the same height as the most prominent structure. Light units should be placed on the structures to ensure that the lights are visible to a pilot approaching from any direction. If one or more of the structures within the group are a solid mass (nonskeletal), additional lighting may be necessary to make sure that the light is not being blocked by the more prominent structure(s). For the purpose of marking and lighting these structures, a group of obstructions is considered to be three or more structures.
6.8 Special Cases.
When lighting systems are installed on structures located near highways, waterways, airport approach areas, etc., caution should be exercised to ensure that the lights do not distract or otherwise cause a hazard to motorists, vessel operators, or pilots on an approach to an airport. In these cases, shielding may be necessary and should not derogate the lighting system’s intended purpose.
CHAPTER 7. HIGH-INTENSITY FLASHING WHITE OBSTRUCTION LIGHT
SYSTEMS
7.1 Purpose.
High-intensity (L-856) flashing white obstruction lights provide the highest degree of conspicuity both day and night. Recommendations on lighting structures can vary, depending on terrain features, weather patterns, geographic location, and number of structures.
Use extreme caution when using high-intensity flashing white lights. Using a 24-hour, highintensity flashing white light system in urban/populated areas is not normally recommended due to their tendency to merge with background lighting in these areas at night. This makes it extremely difficult for some types of aviation operations (i.e., medevac) and police helicopters to see these structures. Additionally, this type of system in urban and rural areas often results in complaints.
7.2 Standards.
High-intensity flashing white obstruction lights should be used during daytime with automatically selected, reduced intensities for twilight and nighttime operations. When highintensity white obstruction lights are operated 24 hours a day, other methods of marking and lighting may be omitted. This system should not be recommended on structures 700 feet (213.36
m) AGL or less unless an FAA aeronautical study shows otherwise. The number of light levels needed is shown in, Figures A-13 and A-14.
7.3 Control Device.
Light intensity is controlled by a device (photocell) that changes the light’s intensity when the ambient light changes. The illuminance sensing device should, if practical, face the northern sky in the Northern Hemisphere and the system should automatically change intensity steps when the illuminance reaching a north-facing vertical surface is as follows:
1. Day-to-twilight. This should not occur before the illumination drops to 60 foot-candles
(645.83 lux) but should occur before it drops below 35 foot-candles (376.74 lux).
2. Twilight-to-night. This should not occur before the illumination drops below 5 foot-candles
(53.82 lux) but should occur before it drops below 2 foot-candles (21.53 lux).
3. Night-to-day. The intensity changes listed in subparagraphs 7.3.1 and 7.3.2 above should be reversed when changing from the night-to-day mode
7.4 Units per Level.
One or more light units are needed to obtain the desired horizontal coverage. The number of light units recommended per level (except for the supporting structures of catenary wires and
buildings) depends upon the average outside diameter of the specific structure and the horizontal beam width of the light fixture. Light units should be installed to ensure an unobstructed view of the system by a pilot approaching from any direction. The number of lights recommended below is the minimum.
1. Structures 20 feet (6.10 m) or less in diameter. Three light units per level.
2. Structures exceeding 20 feet (6.10 m) but not more than 100 feet (30.48 m) in diameter. Four light units per level.
3. Structures exceeding 100 feet (30.48 m) in diameter. Six light units per level.
4. Structures exceeding 200 feet (60.96 m) in diameter. Eight light units per level.
7.5 Installation Guidance.
On most obstruction high-intensity light fixtures, the effective peak intensity of the light beam can be adjusted from zero-to-eight degrees above the horizon. Standard installation should place the top light at zero degrees to the horizontal and all other light units installed in accordance with Table 7-1.
Table 7-1. Light Unit Elevation Above the Horizontal
Height of Light Unit Above Terrain
Degrees of Elevation Above
Exceeding 500 feet AGL
Above 400 feet to 500 feet AGL
Above 300 feet to 400 feet AGL
300 feet AGL or less
1. Vertical Aiming. When terrain, nearby residential areas, or other situations dictate, the light beam may be further elevated above the horizontal. The main beam of light at the lowest level should not strike the ground closer than 3 SM (4.83 kilometers (k)) from the structure.
If additional adjustments are necessary, the lights may be individually adjusted upward, in one-degree increments, starting at the bottom. Excessive elevation may reduce its conspicuity by raising the beam above a collision course flight path.
2. Relocation or Omission of Light Units. Light units should not be installed in such a manner that the light pattern/output is disrupted by the structure.
a. Lowest Level. The lowest level of light units may be installed at a higher elevation than normal on a structure if the surrounding terrain, trees, or adjacent building(s) would obscure the lights. In certain instances, as determined by the FAA, the lowest level of lights may be eliminated.
b. Two Adjacent Structures. When two structures are within 500 feet (152.40 m) of each other and the light units are installed at the same levels, the sides of the structures facing each other do not need be lighted (see Figures A-11 and A-12). However, all lights on both structures must flash simultaneously, except for adjacent catenary support structures.
Vertical placement of the lights should be adjusted to either or both structures’ intermediate levels to place the lights on the same horizontal plane. If one structure is higher than the other, a complete level of lights should be installed on the higher structure that extends above the top of the lower structure. If the structures are of such heights that the levels of lights cannot be placed in identical horizontal planes, then the light units should be placed so that the center of the horizontal beam patterns do not face toward the adjacent structure. For example, structures situated north and south of each other should have the light units on both structures installed on a northwest/southeast and northeast/southwest orientation.
c. Three or More Adjacent Structures. The treatment of a cluster of structures as an individual or a complex of structures will be determined by the FAA, taking into consideration the location, heights, and spacing of other structures.
7.6 Radio and Television Towers and Similar Skeletal Structures.
1. Mounting Lights.
The number of levels recommended depends on the height of the structure, excluding antennas and similar appurtenances. At least three lights should be installed on each level and mounted to ensure that the effective intensity of the full horizontal beam coverage is not impaired by the structural members.
2. Top Level.
One level of lights should be installed at the highest point of the structure. If the highest point is a rod or antenna incapable of supporting a lighting system, then the top level of lights should be installed at the highest portion of the main skeletal structure. If guy wires come